For both handheld and larger yet still "portable" units more appropriate for automotive use, I've always preferred Garmin - I've had four Garmin units so far (GPS45, StreePilot1, StreetPilotIII, Etrex Legend). Currently, I've got the Etrex Legend for bicycling and hiking and an older StreetPilot III for the Durango. None of *my* Garmin units are touch-screen (and I'm quite happy with not cleaning fingerprints off of the screen). However the current Garmin 26XX (2610, 2620, others) units *are* touch screen, and likely what you might be looking for.
HOWEVER, note that if you're looking for a unit for the Viper, note that most of the automotive units have some issues fitting *on* the dash, due to the very short distance between the dash and the windshield. You might also think about your impression when first driving the Viper - remember how you felt blind to the right front, due to the low rearview mirror? Well, it gets worse if you stick a GPS unit on the dash which will block just as much, if not more, of your view to the right front - depending on your height.
Thus, you might want to consider a built-in unit, with a larger slide-out screen which sits below the top of the dash (and only blocks the center vent on Gen IIs, which is no big deal). I recommend the Pioneer AVIC-N1 (and now, AVIC-N2). But, its not a portable, and not an insignificant installation (I spent about 8 hours installing mine). Its a touch screen unit (*not* necessarily a plus in my book), and a very nice CD/DVD/MP3/etc player. I find the bigger screen much nicer than any of the portable units, and I didn't have to make a custom mounting bracket, as I've had to do for all of the other portable units I've used. The only real drawback of the Pioneer unit is the ridiculously small memory - you have to load the DVD to figure any routes - but you can load a "home area" into flash memory and it will show a detailed screen for most driving, say, within 10 miles of home for example.
I know of friends that have picked up a GPS unit sold by Costco (Lowrence?, don't remember), and was very happy with that unit also, which is also touch screen, so definitely check around.